DE20101605U1 - Device for providing an input signal for a line mismatched on the output side - Google Patents

Description

OOIC 0516DEG

iC-Haus GmbH

Device for providing an input signal for a line with a mismatch on the output side

The invention relates to a device for providing an input signal for a line that is mismatched on the output side, and to a line driver for use in such a device.

For data transmission over a multi-wire line, especially a two-wire line, line drivers are usually used that can deliver a sufficiently high output current. Such line drivers are well known.

A line driver is usually connected via its output resistance to the input of a line whose characteristic impedance is much smaller than the line's terminating resistance. The output resistance, on the other hand, corresponds approximately to the characteristic impedance of the line. Such an arrangement, the functioning of which is explained in more detail below, is shown in Fig. 1. A disadvantage of conventional line drivers is that a relatively high power loss is generated in the output resistance of the line driver. The power loss depends, among other things, on the voltage applied to the input of the output resistance and the voltage occurring at the input of the line, which is influenced by signal components reflected at the line output.

The invention is based on the object of providing a device for providing an input signal for a line which is mismatched on the output side and a line driver suitable for this purpose, with which the power loss in the output resistance of the line driver can be significantly reduced compared to a conventional line driver arrangement.

The core idea of the invention is to develop a line driver that generates a voltage curve at the input of the output resistor of the line driver that approximates the voltage curve at the input of a line that is preferably matched on the input side and is influenced by signal components reflected at the end of the line. In this way, the time during which a current flows through the output resistor and thus the power loss in the output resistor of the line driver can be significantly reduced.

The invention solves the above-mentioned technical problem with the features of claim 1.

For this purpose, the device for providing an input signal for a line that is mismatched on the output side has a line driver that is connected to the input of the line via a resistor, also referred to below as the output resistor. Furthermore, a control device is implemented in the line driver that can control a first switch for applying a supply voltage to the input of the resistor, a second switch for applying a reference potential to the input of the resistor and a third switch for applying a first auxiliary voltage to the input of the line at predeterminable times depending on a data input signal in order to minimize the power loss in the resistor.

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At this point it should be mentioned that the term "output-side mismatched line" preferably refers to a line that is terminated with a terminating resistor that is greater than the characteristic impedance of the line. For example, a 1 kOhm resistor is connected. However, smaller resistors can also be used.

In order to be able to reduce the power loss in the resistor as much as possible, it is necessary to determine the time at which the data input signal reflected at the output of the line appears at the input of the line. Knowing this time, it is then possible to open the third switch immediately before the reflected signal arrives at the input of the line. This is because the time between the opening of the third switch and the arrival of the reflected signal at the beginning of the line determines the time during which a current flows through the resistor and thus causes a power loss in the resistor. For this purpose, the line driver has a current or voltage detector connected to the input of the line and an evaluation device which determines the times for opening and closing the first, second and third switches depending on the data input signal and the output signal supplied by the current or voltage detector.

As long as the line driver is operated in a cool environment, the heat development in the resistor does not affect the functionality of the power driver. The line driver can therefore be operated at a correspondingly low component temperature like a conventional line driver, i.e. without controlling the third switch. However, if a certain critical component temperature is exceeded, the line driver can be damaged or

even be destroyed. This can be avoided by minimizing the power loss in the resistor when a critical temperature is exceeded. To do this, as already mentioned, the third switch is used to switch on the first auxiliary voltage. With the help of a temperature sensor, it is now possible to determine when a critical component temperature has been exceeded. In response to the critical temperature being exceeded, the control device then controls the third switch as well as the first and second switches accordingly.

It is advisable for the resistance at the input of the line to be equal in magnitude to the characteristic impedance of the line.

Since, in the case of a mismatched line on the output side, the voltage on the output side is temporarily approximately twice as high as the input voltage, the first auxiliary voltage is essentially equal to half the supply voltage in order to prevent the output voltage of the line from exceeding the supply voltage and to compensate connected loads.

The power loss in the resistor can be further reduced by, for example, applying a second auxiliary voltage to the input of the line via a fourth switch under the control of the control device at predetermined times. The first auxiliary voltage is expediently greater than half the supply voltage, whereas the second auxiliary voltage assumes a value below half the supply voltage in order to optimize the voltage curve at the end of the line. In this way, a distorted, step-shaped voltage curve can be generated at the input of the line, which, together with the voltage curve at the input of the resistor, further reduces the current through the resistor. This

Voltage drops along the line are compensated.

The third switch can be connected to the reference potential via a capacitor. The capacitor is used to feed a predetermined current into the line and to take it back from it.

Advantageously, the control device, the first, second and third and, if applicable, further switches, the current or voltage detector, the temperature sensor, the evaluation device and the resistor are constructed as an integrated circuit.

The technical problem is also solved with the features of claim 7, which relates to a line driver.

The invention is explained in more detail below using an embodiment in conjunction with the accompanying drawings. They show:

Fig. 1 shows a conventional line driver connected to a mismatched line on the output side;

Fig. 2a - 2d the voltage curves at four different reference points between the input of the

line driver and the output of the line; Fig. 3 the current flow through the output resistance of the line driver; Fig. 4 the power loss occurring in the output resistance;

Fig. 5 shows the line driver according to the invention, which is connected to a line with a mismatch on the output side;

Fig. 6a - 6d the voltage waveforms at four reference points between the input of the line driver and

the output of the line; Fig. 7a - 7c the control signals for closing and opening the

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first, second and third switches;

Fig. 8 the current flow through the closed third switch, and

Fig. 9 the power loss occurring in the output resistance.

Fig. 1 shows a conventional line driver 20', which was already mentioned at the beginning and is connected to a line 10' via an output resistor 30' at the input 3. The line 10' is terminated with a terminating resistor 80', which is much larger in magnitude than the characteristic impedance of the line 10'. The line driver 20' has a first switch 40' for applying a supply voltage VB to the input 2 of the output resistor 30'. Furthermore, a second switch 50' is provided in the line driver 20', via which the input 2 of the output resistor 30' is connected to ground. The resistance value of the output resistor 30' is expediently equal to the characteristic impedance of the line 10'.

The operation of the known line driver can best be explained in connection with the curves shown in Figures 2a - 2d and Figures 3 and 4.

Fig. 2a shows the data signal which is applied to the input 1 of the line driver 20' for transmission via the line 10'. The data signal comprises, for example, a sequence of zeros and ones, which can correspond to the low or high level of the data signal. While a one is present at the input 1 of the line driver 20', the supply voltage VB is applied to the input of the resistor 30' via the switch 40', as shown in Fig. 2b.

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is shown. Since the output resistor 30' and the line 10' act like a voltage divider, half the supply voltage is applied to the reference point 3 for a time of 2td, which corresponds to twice the propagation time of the line 10'. As a result of the mismatch on the output side, the data signal transmitted via the line 10' is partially reflected. After a time of 2td, the voltage at the input 3 of the line 10' therefore rises suddenly to the supply voltage. At the end of the data signal, the switch 40' is opened again and switch 50' is closed. At this point in time, the voltage drops to the reference potential, whereas the voltage at the input 3 of the line 10' drops to half the supply voltage for a time of 2td and then to the reference potential. A comparison of the voltage curves according to Fig. 2b and 2c shows that after the supply voltage is applied to the reference point 2 and after the supply voltage is disconnected, a voltage drops across the output resistor 30' for a time 2td, which causes the current flow in the output resistor 30' shown in Fig. 3. The corresponding power loss in the output resistor 30' is shown in Fig. 4. The output signal at the reference point 4, which is delayed by the transit time td of the line 10' compared to the data input signal, is shown in Fig. 2d.

With the arrangement shown in Fig. 5, it is now possible to reduce the current flow through the output resistor 30 and thus the power loss compared to the conventional power driver shown in Fig. 1.

The line driver 20 shown in Fig. 5 is initially connected to the input 3 of a line 10 via an output resistor 30. The line 10 is terminated at the output 4 with a terminating resistor 80, the value of which is much larger than the characteristic impedance of the line 10. With

In other words, there is a significantly mismatched line 10 on the output side. The line driver 20 has a current or voltage detector 22 which is connected to the input 3 of the line 10. The current or voltage detector 22 is connected to an evaluation device 23, the function of which is explained in detail below. Furthermore, a control device 21 of the line driver 20 is connected to a first switch 40 for applying the supply voltage to the input 2 of the resistor 30, to a second switch 50 for applying a reference potential, in particular ground, to the input 2 of the resistor 30 and to a third switch 60 for applying a first auxiliary voltage VH1 to the input 3 of the line 10. Alternatively, the control device 23 can be connected to further switches for applying various auxiliary voltages. In Fig. 5, only a further switch 62 for applying a second auxiliary voltage VH2 is shown in dashed form. The switches 60 and 62 are connected to ground via a capacitor 90 and 92 respectively.

In the further explanation of the line driver 20 according to the invention, only the switch 60 and the first auxiliary voltage are taken into account. Furthermore, the line driver 20 has a temperature sensor 24, via which the temperature of the line driver 20 can be measured.

The temperature sensor 24 is preferably connected directly to the control device 21 in order to determine the switching times of the switches 40, 50 and 60, as will be explained in more detail below.

The functionality of the line driver 20 is explained in more detail below in conjunction with Figures 6a - 6d, 7a - 1c and Figures 8 and 9.

As already mentioned, the line driver 20 according to Fig. 5 is able to use the switch 60 and the associated

connected auxiliary voltage, the power loss in the output resistor 30 can be significantly reduced compared to the power loss in the resistor 30' of the conventional line driver according to Fig. We first consider Figures 7a - 7c, which show the control signals provided by the control device 21 for closing and opening the switches 40, 50 and 60 respectively. With the rising edge of the data signal shown in Fig. 6a, i.e. at time t 1, the switch 60 is closed and thus the auxiliary voltage VHl is applied to the input 3 of the line 10. The auxiliary voltage VHl is preferably half the supply voltage VB of the line driver 20. The switches 40 and 50 are open at this time. Up to time t2, which is shown in Fig. 7c, the switches 40 and 50 are still open, while the switch 60 remains closed during this time. As shown in Fig. 6c, the time period between ti and t2 is shorter than twice the running time 2td of line 10. As long as switch 60 is closed, the auxiliary voltage VHl, in this example half the supply voltage, is present at both input 2 of output resistor 30 and input 3 of line 10. This situation is shown in Figures 6b and 6c. Consequently, no current flows through output resistor 30 during this time, so that no power loss occurs in output resistor 30. This is illustrated in Fig. 9. At time t2, switch 60 is now opened and switch 40 is closed, causing the voltage at input 2 of output resistor 30 to rise abruptly to the level of supply voltage VB. At this moment, a potential difference of approximately VB/2 is present at output resistor 10, specifically at points 2 and 3, which causes current to flow through output resistor 30. The corresponding

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Power loss in the output resistor 30 is shown in Fig. 9. However, as soon as the data signal reflected at the output 4 of the line arrives at the input 3 of the line 10 after the time 2td (time t3), measured from the time ti, the voltage at the line input 3 increases abruptly to the supply voltage. The potential difference at the reference points 2 and 3 and thus the power loss in the output resistor 30 then approaches zero again. At the end of the data signal at the time t4, the switch 40 is opened, the switch 50 is still open and the switch 60 is closed again. At the time t0, the switch 50 is closed and the switch 60 is opened. At this point it should be emphasized that the time period t5-t4 is less than twice the running time 2td of the line 10. The switching processes are shown in detail in Figs. 7a to 7c. As can be seen from Figs. 6b and 6c, the opening of switch 40 and the closing of switch 60 at time t4 causes the voltages at reference points 2 and 3 to simultaneously drop from VB to VB/2.

This voltage curve is maintained until time t5. After closing switch 50 and opening switch 60 at time t4, however, the voltage at reference point 2 drops abruptly to the reference potential, while the voltage at the reference point only drops to the reference potential after time t4 + 2td (corresponds to time t6) due to the data signal reflected at the end of the line. The reason for this is that the data signal reflected at the end of the line appears at line input 3 until time t6. Accordingly, the time duration t3-t2 and t6 - t5, which is the time between opening switch 60 and the arrival of the reflected signal at input 3 of line 10, or the time between opening switch 60 and the decay of the

reflected data signal at the input of line 10, the time during which power loss is generated in the output resistor 30. The power loss generated in the output resistor 30 is shown in Fig. 9. In order to minimize the power loss in the output resistor 30, the switch 60 should therefore only be opened shortly before the reflected signal appears at the input 3 at time t3 or shortly before the reflected signal at the input of line 10 decays at time t6. In order to be able to determine the times t3 and t6 entered in Fig. 6c, the current or voltage detector 22 is connected to the line input 3.

At the beginning of the data transmission, the first data bit to be transmitted is expediently used to determine these points in time. Depending on the data signal according to Fig. 6a and the signal curve detected at reference point 3 by the current or voltage detector 22, the evaluation device 23 determines the switching point ti for closing the switch 60, the point in time t2 for opening the switch 60 and closing the switch 40, the point in time t4 for opening the switch 40 and for closing the switch 60 again, and the point in time t5 for opening the switch and closing the switch 40.

Fig. 8 shows the current flowing from the capacitor 90 into the line 10 and vice versa from the line 10 back into the capacitor 90 when the switch 60 is closed.

Fig. 6d shows, similar to Fig. 2d, the voltage curve at the output 4 of the line 10, which is delayed by the propagation time t _d of the line 10 compared to the input signal.

Usually, the heat loss generated in the output resistor 30 does not lead to damage or destruction of the line driver 20, provided the component temperature is sufficiently low. Therefore, in non-critical

Temperatures of the line driver 10 according to Fig. 5 can initially be operated without controlling the switch 60. However, as soon as the temperature sensor 24 detects that a predetermined critical temperature has been exceeded, the control device 21 is prompted to control the switch 60 during the next switching cycle in order to reduce the power loss in the output resistor 30 and thus the temperature of the line driver 20 again. In other words, the line driver 20 according to Fig. 5 can be operated at non-critical component temperatures like a normal line driver 20' shown in Fig. 1. The switch 60 is only controlled when a critical temperature is exceeded.

A comparison of Figures 4 and 9 shows that in the conventional line driver 20', heat loss is generated in the output resistor 30' during a time period of 2td, while in the line driver according to Figure 5, when using the switch 60, power loss is generated in the output resistor 30 only between the time t3-t2 and t6-t5.

Claims (9)

1. Device for generating an input signal for a line ( 10 ) which is mismatched on the output side, comprising a line driver ( 20 ) which is connected to the input of the line ( 10 ) via a resistor ( 30 ) and has a control device ( 21 ) which, depending on a data input signal, can control a first switch ( 40 ) for applying a supply voltage to the input of the resistor ( 30 ), a second switch ( 50 ) for applying a reference potential to the input of the resistor ( 30 ) and a third switch ( 60 ) for applying a first auxiliary voltage to the input of the line ( 10 ) at predeterminable times in order to minimize the power loss in the resistor ( 30 ). 2. Device according to claim 1, characterized in that the line driver ( 20 ) has a current or voltage detector ( 22 ) connected to the input of the line ( 10 ) and an evaluation device ( 23 ) which determines the times for opening and closing the first, second and third switches as a function of the data input signal and the output signal supplied by the current or voltage detector. 3. Device according to claim 1 or 2, characterized in that
the line driver ( 20 ) has a temperature sensor ( 24 ), and that
the control device ( 21 ) only activates the third switch when the temperature sensor ( 24 ) detects that a predetermined temperature has been exceeded. 4. Device according to one of claims 1 to 3, characterized in that the resistor ( 30 ) corresponds to the characteristic impedance of the line ( 10 ), and that the line ( 10 ) is terminated with a resistor which is greater than the characteristic impedance of the line ( 10 ). 5. Device according to one of claims 1 to 4, characterized in that the first auxiliary voltage is substantially equal to half the supply voltage. 6. Device according to one of claims 1 to 4, characterized in that the control device ( 21 ) can control a fourth switch for applying a second auxiliary voltage to the input of the line ( 10 ) at predeterminable times, the first auxiliary voltage assuming a value between the supply voltage and half the supply voltage and the second auxiliary voltage assuming a value between reference potential and half the supply voltage. 7. Device according to one of claims 1 to 6, characterized in that the power driver ( 20 ) and the resistor ( 30 ) are designed as an integrated circuit. 8. Device according to one of claims 1 to 7, characterized in that the third switch ( 60 ) is connected to reference potential via a capacitor ( 90 ) 9. Line driver for connecting to a mismatched line ( 10 ) on the output side, comprising
a resistor ( 30 ) connectable to the input of a line ( 10 ), and
a control device ( 21 ) which, depending on a data input signal, can control a first switch ( 40 ) for applying a supply voltage to the input of the resistor ( 30 ), a second switch ( 50 ) for applying a reference potential to the input of the resistor ( 30 ) and a third switch ( 60 ) for applying a first auxiliary voltage to the output of the resistor ( 30 ) at predeterminable times in order to minimize the power loss in the resistor ( 30 ).